The present application relates to anodic electrochromic materials. More particularly, the present invention relates to coupled anodic electrochromic compounds wherein two or more monomeric electrochromic compounds are coupled to give a new anodic compound with improved properties over the monomeric electrochromic compounds.
Electrochromic devices, and electrochromic media suitable for use therein, are the subject of numerous U.S. patents, including U.S. Pat. No. 4,902,108, entitled xe2x80x9cSingle-Compartment, Self-Erasing, Solution-Phase Electrochromic Devices, Solutions for Use Therein, and Uses Thereofxe2x80x9d, issued Feb. 20, 1990 to H. J. Byker; Canadian Pat. No. 1,300,945, entitled xe2x80x9cAutomatic Rearview Mirror System for Automotive Vehiclesxe2x80x9d, issued May 19, 1992 to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled xe2x80x9cVariable Reflectance Motor Vehicle Mirrorxe2x80x9d, issued Jul. 7, 1992 to H. J. Byker; U.S. Pat. No. 5,202,787, entitled xe2x80x9cElectro-Optic Device:, issued Apr. 13, 1993 to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled xe2x80x9cControl System For Automatic Rearview Mirrorsxe2x80x9d, issued Apr. 20, 1993 to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled xe2x80x9cTinted Solution-Phase Electrochromic Mirrorsxe2x80x9d, issued Jan. 11, 1994 to D. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled xe2x80x9cUV-Stabilized Compositions and Methodsxe2x80x9d, issued Jan. 18, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled xe2x80x9cVariable Reflectance Mirrorxe2x80x9d, issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No. 5,294,376, entitled xe2x80x9cBipyridinium Salt Solutionsxe2x80x9d, issued Mar. 15, 1994 to H. J. Byker; U.S. Pat. No. 5,336,448, entitled xe2x80x9cElectrochromic Devices with Bipyridinium Salt Solutionsxe2x80x9d, issued Aug. 9, 1994 to H. J. Byker; U.S. Pat. No. 5,434,407, entitled xe2x80x9cAutomatic Rearview Mirror Incorporating Light Pipexe2x80x9d, issued Jan. 18, 1995 to F. T. Bauer et al.; U.S. Pat. No. 5,448,397, entitled xe2x80x9cOutside Automatic Rearview Mirror for Automotive Vehiclesxe2x80x9d, issued Sep. 5, 1995 to W. L. Tonar; and U.S. Pat. No. 5,451,822, entitled xe2x80x9cElectronic Control Systemxe2x80x9d, issued Sep. 19, 1995 to J. H. Bechtel et al., each of which patents is assigned to the assignee of the present invention and the disclosures of each of which are hereby incorporated herein by reference, are typical of modern day automatic rearview mirrors for motor vehicles. These patent references describe electrochromic devices, their manufacture, and electrochromic compounds useful therein, in great detail.
While numerous electrochromic devices are possible, the greatest interest and commercial importance are associated with electrochromic windows, electronic displays, light filters and mirrors. A brief discussion of these devices will help to facilitate an understanding of the present invention.
Electrochromic devices are, in general, prepared from two parallel substrates coated on their inner surfaces with conductive coatings, at least one of which is transparent such as tin oxide, or the like. The two substrates of the device are separated by a gap or xe2x80x9ccavityxe2x80x9d, into which is introduced the electrochromic medium. A commercially available electrochromic medium typically contains a solvent and at least one anodic and/or cathodic electrochromic compound which changes color upon electrochemical oxidation or reduction. Upon application of a suitable voltage between the electrodes, the electrochromic compounds are oxidized or reduced depending upon their redox type, changing the color of the electrochromic medium. The electrochromic compounds change from a colorless or near colorless state to a colored state. Upon removal of the potential difference between the electrodes, the electrochemically activated redox states of electrochromic compounds react, becoming colorless again, and xe2x80x9cclearingxe2x80x9d the device.
In electrochromic mirrors, devices are constructed with a reflecting surface located on the outer surface of the substrate which is most remote from the incident light (i.e. the back surface of the mirror), or on the inner surface of the substrate most remote from the incident light. Thus, light striking the mirror passes through the front substrate and its inner transparent conductive layer, through the electrochromic medium contained in the cavity defined by the two substrates, and is reflected back from a reflective surface as described previously. Application of voltage across the inner conductive coatings results in a change of the light reflectance of the mirror.
In electrochromic devices, including windows and mirrors, the selection of the components of the electrochromic medium is critical. The medium must be capable of reversible color changes over a life cycle of many years, including cases where the device is subject to high temperatures as well as exposure to ultraviolet light.
A variety of anodic electrochromic compounds are available, among which are the 5,10,-dihydrophenazines, their phenothiazine analogs, and both ring-substituted as well as heteroatom-substituted derivatives. For example, 5,10-dimethyl-5,10-dihydrophenazine: 
is a well known electrochromic compound. When this compound is oxidized to the 1+ oxidation state, the compound exhibits a weak absorption band at xcx9c700 nm and a more intense, but still modest, absorbance at xcx9c450 nm in the visible region of the visible spectrum.
The compound can be oxidized at higher potentials to the more highly oxidized 2+ species, which is more susceptible to both thermal and photo degradation. Moreover, some 2+ species can exist even in devices where the applied voltage is well controlled and less than that required for direct oxidation, (e.g. by disproportionation of 1+ species).
Heretofore, electrochromic devices have not found wide acceptance as architectural windows, where darkening during daylight hours (thus being subject to UV exposure in their activated state) is a frequent occurrence. Electrochromic devices used in these environments have shown a tendency to degrade over time, even when UV absorbing coatings and additives are used in attempts to mitigate these effects. Thus the need exists for electrochromic devices that have the stability desired for applications such as architectural windows and glazings for automobiles. Additionally it is desirable to obtain anodic electrochromic materials with more intense absorbances in the visible as well as absorbances in the near-infrared regions of the electromagnetic spectrum.
It has now been surprisingly discovered that properly bridging two or more anodic monomeric electrochromic compounds can afford coupled electrochromic compounds with electrochemically activated forms that have enhanced photochemical stability. Such coupled electrochromic compounds also have electrochemical properties and absorption properties different than their uncoupled analogs, and in many cases, are found to exhibit absorbance in the near-infrared region of the spectrum.